Pressure-sensitive adhesive and adhesive tape



Patented May 22, 1951 PRE SSURE-SENSITEVE ADHESIVE AND ADHESIVE TAPEClarence Joseph Ebel, St. Paul, Minn, assignor to Minnesota. Mining &Manufacturing Company, St. Paul, Minn, a corporation of Delaware NoDrawing.

Application February 16, 1948,

Serial No. 8,752

Claims. 1.

The present application is a continuation-inpart of my copendingapplication Serial No. 589,277, filed April 19, 1945, now abandoned.

This invention relates to eucohesive adhesives which are normally tackyand pressure-sensitive, and to pressure-sensitive adhesive tapes madetherefrom. In particular, the invention is concerned with adhesives, ofthe type described, having a saturated oxygen-containing syntheticrubbery vinyl polymer base. These adhesives can be coated on cloth,paper or non-fibrous film backings, for example, to provide highlyeffective pressure-sensitive adhesive sheets and tapes which have beenfound useful in the arts of painting and lacquering, package sealing,mending of books and records, and the like. The tapes are aggressivelyand stably tacky and instantly adhere upon contact to most surfaces,without need of activation by moistening or heating.

By eucohesive it is meant that the tacky adhesive mass is more cohesivethan adhesive such that offsetting or transfer of adhesive material doesnot result when a sheet or tape coated therewith is unwound from rollsor removed from surfaces to which temporarily applied, and that suchcoated sheet or tape can be handled without transfer of adhesivematerial to the fingers.

Many vinyl polymers, such for example as polymerized vinyl acetate, havebeen found useful in adhesive compositions for application from solutionor dispersion. The adhesive dries to a hard, firm and non-tacky state,and may usually be reactivated by heat or solvents. These hard polymershave been used as permanent cementing layers between varioustypes ofsurfaces, but have not been found suitable for the preparation ofnormally tacky and eucohesive pressure-sensitive adhesives.

Other vinyl polymers, and specifically, the rubbery polymers obtained bythe polymerization of vinyl monomers having longer side chains, aresofter than the polymers having shorter side chains, such as thepolymers of vinyl acetate, methyl acrylate, methyl methacrylate, etc.Some of these softer polymers have been found to be normally tacky, andhence have been suggested as adhesive coatings for normally tacky andpressure-sensitive adhesive tapes.

However, the normally tacky vinyl polymers have not given completesatisfaction when tested as pressure-sensitive adhesives. For example,the vinyl polymers are in many cases low in adhesion, so that theresulting coated tape does not adhere with sufiicient tenacity tosurfaces to which applied. The addition of tackifiers such is polyvinylbutyrate.

as rosin, various derivatives of rosin suchas estergum or hydrogenatedrosin, terpene resins, etc, improves the adhesion but at the same timesoftens the composition so that it is no longer sufficiently eucohesive.Other vinyl polymers have been found to be eucohesive and sufiicientlytacky at room temperature, but are temporarily, and sometimespermanently, softened and weakened excessively on moderate heating.

These and other deficiencies of the specific class of polymers hereinvolved, with respect to their usefulness as normally tacky andpressure sensitive adhesives, are overcome by my invention. I providenew adhesive compositions, as hereinafter more fully defined andidentified, having superior qualities for use in masking tape, mendingtape, electrical insulating tape, and other forms of coated normallytacky and pressure-sensitive adhesive tapes.

I utilize oxygen-containing synthetic rubbery vinyl polymer basematerials of the class consisting of polymers of esters of acrylic acid,esters of vinyl alcohol, and others of vinyl alcohol; having saturatedterminal side chains of at least four carbon atoms with each ofsuch'terminal side chains linked to a single carbon atom through asingle oxygen atom. The polymer can be a copolymer of the specifiedmonomer and a minor proportion of another polymerizable monomer servingas a modifier. An illustration of such polymer is a copolymer of '75parts of 2-ethylbutyi acrylate and 2-5 parts of ethyl acrylate. Anillustration of a polymer of an ester of vinyl alcohol An illustrationof a polymer of an ether of vinyl alcohol is polyvinyl-nbutyl-ether.

I obtain my novel product and result by adding to the saturatedoxygen-containing synthetic rubbery vinyl polymer adhesive base materiala compatible oil-soluble heatandoil-'reactive phe nol-aldehyde resin, asillustrated in the follow,- ing examples.

EXAMPLE 1 Parts by Weight Rubbery acrylate copolymer 100 Titaniumdioxide pigment 5 Phenol-aldehyde resin 5 Ester gum 10 Zinc resinate lHeptane 300 Ethyl alcohol s 6 The vinlyl polymer used in this examplewas prepared from parts of Z-ethylbutyl acrylate and 25 parts of ethylacrylate, copolymerized in aqueous dispersion and coagulated with alum;

the final washed and dried polymer was of about the consistency oflightly milled crude crepe rubber. Coagulation with barium chloride hasprovided a slightly more soluble and more tacky polymer, which, however,was difiicult to obtain free of chlorides for use in makingchloride-free adhesive tapes.

The phenol-aldehyde resin of the above formula was prepared fromapproximately two mols of formaldehyde and one mol ofpara-tertiary-amylphenol, using an alkaline catalyst, according to theprocedure given below. It may be classified as an unmodified oil-solublealkaline-condensed heat and-oil-reactive phenol-aldehyde resin. It wascompatible with the vinyl polymer.

The polymer, pigment, and rosin derivative resins were mixed together ina steam-jacketed heavy-duty internal mixer until smooth; thephenol-aldehyde resin was then added and mixing continued withoutheating for 30 minutes, after which 80 lbs. steam was applied in thejacket of the mixer for two hours. The temperature of the batch duringthis period was of the order of JAG-150 C. Cooling to below the boilingpoint of the solvent, and dilution with heptane followed by addition ofthe alcohol to reduce the viscosity of the final solution, compieted thepreparation of the adhesive. The solution had a final viscosity of5000-7000 centipoises at 80 F., and was satisfactorily coated on variousbackings, including cloth backings, to produce tacky and adherent butfirm and long-aging pressure-sensitive adhesive sheets and tapes.

When this same acrylate copolymer was used without the phenol-aldehyderesin for the production of pressure-sensitive adhesives, it wasnecessary to reduce the amount of rosin type tackifier resins, or evento eliminate them entirely, in order to prevent splitting of theadhesive layer on unwinding of the roll or removal of the tape fromsurfaces to which it had been applied; and this was particularly truewhere the tape was to be used at slightly elevated temperatures.Elimination of the resins resulted in an adhesive, and therefore anadhesive tape or sheet, having much reduced tackiness or adhesion valueunder normal conditions of use. The treated product of the presentexample, on the other hand, is satisfactorily eucohesive even at wellabove room temperatures, and furthermore has ample tackiness over a widerange of conditions and for a wide variety of uses.

As above prepared, the adhesive is already in its final eucohesive statewhen applied to the backing. By a variation of the method of proceduregiven. it is possible to coat the adhesive prior to heating, and laterto convert it to the eucohesive state when in the coated sheet form. Forexample, in the preparation of the adhesive of the present example, thebatch in the internal mixer may be kept cool, by passing cooling watercontinuously through the mixer jacket, during the entire mixing anddissolving cycle. Alternatively, the pigment may be milled into thepolymer on a rubber mill, and the milled base, together with theresinous ingredients, then dissolved and blended in the mixed solventsby means of a paddle type mixer. The adhesive is then coated on asuitable heat-resistant backing; the solvent is removed by evaporation;the coated sheet is wound up in roil form; and the roll is heated in anoven for 2 hours at 300 F. On comparing such tape with the unheatedproduct, and also with a similarly heated prodnot in which thephenol-aldehyde resin was omitted, the following comparative test valueswere obtained; in this specific instance, the backing used was acellulose acetate cloth.

1 Roll could not be unwound, the adhesive had softened on heating andhad flowed into the cloth, cementing the adjacent layers together In avariation of this procedure, the coated sheet may be dried in an ovenand heated further in sheet form by electrical or by direct heatradiation; the finished sheet is then slit into desired width and woundup in roll form.

The phenol-aldehyde resin used in Example 1 is representative of awell-known class of phenolic resins designed initially for use withdrying oils in the varnish-making industry, and there characterized asoil-soluble and heat-andoil-active phenol-aldehyde resins. Oilsolubility, and apparently also compatibility with the vinyl polymers ofthe present invention, is obtained by the use of substituted phenolssuch as para-tertiary-amyl phenol, para-tertiary-butyl phenol, andpara-phenyl phenol. Formaldehyde is ordinarily preferred as the aldehydeconstituent, and is present in an amount substantially in excess ofequirnolecular proportion as compared to the phenol. The resin is inpartially reacted state, and is ordinarily employed in the form of hard,brittle lumps.

The specific phenolic resin of Example 1 was produced as follows:

Formula Pounds Formaldehyde (37% solution) 1300 Sodium hydroxidesolution parts sod.

hydroxide in 200 parts water) 330 Para-tertiary-amyl-phenol 1345Procedure Mix the sodium hydroxide solution with the formaldehydesolution; cool; add the para-tertiary-amyl-phenol, and react with gentleheating until the free formaldehyde content of the batch is reduced tonot more than 1% by analysis. Add dilute sulfuric acid to a pH of 1-2,at which point the resin separates from the aqueous phase. Draw oi theaqueous phase, and wash the fluid resin with several changes of hotwater, then heat under vacuum until a test drop of the resin forms abrittle pill on cooling to room temperature. Cool rapidly to roomtemperature, to obtain a brittle resinous product having a temporarymelting-point of the order of -190 F. Break the resin into lumps forstorage or use.

EXAMPLE 2 Parts by weight Cross-linked polyvinyl butyrate 84Phenol-aldehyde resin (as in Example 1) 4.2 Zinc resinate 2.5

Solvent as required.

For the synthetic rubbery vinyl, 425 parts of monomeric vinyl butyrateand 2.13 parts of diisoamyl ethylene dimaleate (prepared from ethyleneglycol, maleic anhydride, and isoamyl alcohol by two-stageesterification) were copolymerized in solution in 425 parts of tertiarybutyl and reacting for 23 hours at about 75-78 C.

The reaction was stopped when the percentage of volatile constituents inthe solution, tested at 100 C., had been reduced by polymerization ofthe reactants to 56.2%. The volatile portion was removed by evaporation,and the polymer Was milled on a rubber-mill together with thephenolaldehyde resin (prepared as for Example 1) and the zinc resinate.During the milling, the mixture was heated by heating the mill rollswith steam at 80-90 p. s. i. for about 45 minutes, which resulted in apronounced stiffening of the mass; It was then dissolved in sufiicientAmsco AX solvent, a volatile hydrocarbon solvent having a high aromaticcontent, to produce a solution.

The compounded polymer adhesive, in the form of the solution prepared asabove, was coated on cellophane and the resulting transparent coatedtape was tested against tapes similarly prepared in which the polymeralone constituted the adhesive coating. The crude polymer-could be usedas a pressure-sensitive eucohesive adhee sive for some purposes but waslow in adhesion value and in internal strength. Addition of the zincresinate and phenol-formaldehyde resin by simple solution and withoutheating increased the tackiness but greatly reduced the internalstrength of the adhesive. Heating, as carried out in the presentexample, caused a firming or an increase in internal strength of theadhesive without seriously reducing its ability to adhere to varioussurfaces when applied thereto under light pressure.

EXAMPLE 3 Polyvinyl normal butyl ether 100 Bakelite BR-14634- 7.5 PaleZitro resin l Toluol 450 The polymerized vinyl-n-butyl-ether, a ratherfirm rubbery polymer obtained in the form of beads, was milled forfiveminutes; the Bakelite and Zitro resins were then added on the mill,which was then heated with steam at 80 p. s. i. in the rolls, and mixingwas continued for t?) minutes. The mass was then removed from the mill,dissolved in the toluol by simple a itation, and spread on cellophane toprovide a heatresistant, tacky, eucohesive pressure-sensitive sheet.

The same type of polyvinyl normal butyl ether by itself provides anadhesive coating ofv rather low adhesion value which exhibits a certaindegree of usefulness at normal room temperature but which softensexcessively when subjected to elevated temperatures, for example up to300 F. The mixture of polymer, Bakelite resin, zinc resinate as aboveprepared is higher in adhesion value, and in addition is highlyresistant to the softening effect of high temperature. For example, at308 F. the polymer alone became too soft to test further after only onhour, while theabove adhesive was notmeasurably softened at 6 hours andwas still usefully firm after 24 hours of heating.

Bakelite BRA 634 is a commercial resin, available at the date of thisapplication, which is an oil-soluble heat-and-oil-reactivephenolaldehyde resin, compatible with the vinyl polymer. It isunderstood to be a para-tertiarybutyl-phenol formaldehyde resin madewith an alkaline catalyst and with more than one mol,

i. e., between 1.5 and 2.0 mole, of formaldehyde for each mol ofsubstituted phenol.

The Zitro resin is a commercially available pale zinc resinatecontaining 4.8% combined zinc and having a low acid number not greaterthan about 15.

In place of the hydrocarbon solvents of Examples 1-3, I may use anyother volatile liquid solvent, e. g. carbon tetrachloride, in which theseveral components may be adequately dissolved or dispersed to a smooth,stable, spreadable form. Alternatively, I may coat my adhesives in theform of aqueous dispersions, prepared either by dispersing the completedadhesive or by mixing aqueous dispersions of polymer, resins, etc. Themixture is coated and dried, and the dried residue is then heated insitu.

The ester gum of Example 1 serves to increase the tackiness or adhesionvalue of the adhesive composition. The ester gum may be omitted and yetthe adhesive will be adequately tacky for some purposes. The proportionmay be increased to 15 parts, or even higher proportions, of ester gumor other tackifiers or mixtures thereof in addition to thephenol-aldehyde resin. For example, one formula which has replaced thatof Example 1 where a transparent and somewhat more tacky adhesive wasrequired, was prepared from parts of the rubbery acrylate copolymer, 5parts of BR-14634 resin, one part of Zitro resin, and 15 parts ofPiccolite. 55-25, a pure hydrocarbon thermoplastic terpene resin havinga melting point of 25 C.

Instead of the 100% phenol-aldehyde resins of the examples,phenol-aldehyde resins formed in the presence of certain modifyingagents, and particularly of modifying agents such as rosin or the like,are found to be useful in combination with my vinyl polymer baseadhesives to provide homogeneous products having increased tackiness aswell as improved firmness and other properties. When used in equivalentamounts based on the phenolic resin content, such modified resins aretherefore to be considered as fully equivalent to the unmodified resinsdescribed above. One such modified resin which is useful both as atackifier and in imparting a desirable degree of internal strength oreucohesiveness to the soft vinyl polymer base may be made, for example,by heating rosin (abietic acid) with an equal weight of a neutralwater-insoluble but still liquid resinous material formed by the partialreaction, in the presence of an alkaline catalyst, of one mol of phenol,four mols of para tertiary butyl phenol, and ten mole of formaldehyde.Heating is controlled so as to provide a homogeneous, hard and brittleresinous product which is still compatible with the rubbery polymeremployed.

The rubbery saturated oxygen-containing vinyl polymers of the exampleswill be seen to have, in each case, relatively long side chains attachedto individual carbon atoms of the carbon skeleton. These side chains ineach case contain at least four carbon atoms, in addition to an oxygenatom.

The specific vinyl polymers herein illustrated have provided excellenttacky and pressure-sensitive adhesives when used in conjunction withspecific amounts of specific phenol-aldehyde resins as stated in theexamples. Other equivalent modifications of such polymers and resins,and other procedures, may require considerable variation from theproportions given. In most cases, the amount of oil-solubleheat-and-oil-reactive 7 phenol-aldehyde resin will fall within theapproximate limits of 2-15 parts by weight of the resin to 100 parts ofthe synthetic oxygen-containing saturated rubbery vinyl polymer.

With the smaller proportions of phenol-aldehyde resin, it is founddesirable to include small amounts of an activator or catalyst, so as todecrease the time and temperature of heating. Zinc resinate is anexample of such an activator which is also a tackifier. efiective as anactivator; when both zinc oxide and rosin are present in thecomposition, zinc resinate is formed. Calcium oxide or hydroxide, andcalcium resinate, are less efiective but still useful activators. Withlarger proportions of phenol-aldehyde resin, the addition of activatorsbecomes of less importance. However, I prefer to include small amountsof zinc resinate or equivalent as indicated in the Examples.

These and other variables will be readily controlled, in accordance withthe teachings of my invention, so as to provide normally tacky andpressure-sensitive adhesive compositions in which a tacky syntheticrubbery oxygen-containing saturated vinyl polymer base adhesive isimproved in internal strength and in heat-resistance by blending andheating with a minor proportion of a compatible, oil-soluble,heat-and-oilreactive phenol-aldehyde resin. There is provided a newadhesive product having improved properties.

Having described various illustrative embodiments of my invention, butWithout intent to be limited thereto, what I claim is as follows:

1. An adhesive composition comprised of a major proportion of asynthetic rubbery polymer of the class consisting of polymers of estersof acrylic acid, esters of vinyl alcohol, and ethers of vinyl alcohol,having saturated terminal side chains of at least four carbon atoms witheach of such terminal side chains linked to a single carbon atom througha single oxygen atom, a minor proportion of a compatible, oil-soluble,heat-and-oil-reactive phenol-aldehyde resin, and a minor proportion ofresinous tackifier including zinc resinate; said adhesive compositionbeing normally tacky and pressure-sensitive when in a solvent-free driedstate.

2. An adhesive composition which in the dry state is a normally tackyand pressure-sensitive eucohesive adhesive adapted for use in pressuresensitive adhesive tape, comprising a heated blend of: (1) 100 parts ofa synthetic rubbery polymer of the class consistin of polymers of estersof acrylic acid, esters of vinyl alcohol, and ethers of vinyl alcohol,having saturated terminal side chains of at least four carbon atoms witheach of such terminal side chains linked to a single carbon atom througha single oxygen atom; (2) about 2-15 parts of a compatible, oilsoluble,heat-and-oil-reactive phenol-aldehyde resin; and (3) a compatibletackifier resin in an amount up to about 16 parts; said heated blendbeing firmer and more heat-resistant than the same blend unheated.

3. An adhesive composition according to claim 2 wherein the syntheticrubbery polymer is an acrylate ester polymer.

4. An adhesive composition according to claim 3 wherein the polymer is acopolymer of three Zinc oxide is highly parts 2-ethy1butyl acrylate andone part ethyl acrylate.

5. Pressure-sensitive adhesive tape comprising a flexible backing membercarrying a coating of an adhesive according to claim 1.

6. Pressure-sensitive adhesive tape comprising a flexible backing membercarrying a coating of an adhesive according to claim 2.

7. Pressure-sensitive adhesive tape comprising a flexible backing membercarrying a coating of an adhesive according to claim 3.

8. Pressure-sensitive adhesive tape comprising a flexible backing membercarrying a coating of an adhesive according to claim 4.

9. A pressure-sensitive adhesive tape wound upon itself in roll form,comprisin a flexible backing and, as a coating thereon, a normally tackyand pressure-sensitive adhesive composition comprised of a majorproportion of a synthetic rubbery polymer of the class consisting ofpolymers of esters of acrylic acid, esters of vinyl alcohol, and ethersof vinyl alcohol, having saturated terminal side chains of at least fourcarbon atoms with each of such terminal side chains linked to a singlecarbon atom through an oxygen atom, and a minor proportion of acompatible, oil-soluble, heat-and-oil-reactive phenol-ald hyde resin.

10. A pressure-sensitive adhesive tape Wound upon itself in roll form,comprising a flexible backing and, as a coating thereon, a normallytacky and pressure-sensitive adhesive composition comprised of a heatedblend of a major proportion of a synthetic rubbery polymer of the classconsisting of polymers of esters of acrylic acid, esters of vinylalcohol, and ethers of vinyl alcohol, having saturated terminal sidechains of at least four carbon atoms with each of such terminal sidechains linked to a single carbon atom through an oxygen atom, and aminor proportion of a compatible, oil-soluble, heat-and-oil-reactivephenol-aldehyde resin, said heated blend being firmer and moreheat-resistant than the same blend unheamd.

' CLARENCE JOSEPH EBEL.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,336,792 Langkammerer et al.Dec. 14, 1943 2,396,098 Haas Mar. 5, 1946 2,410,053 Drew Oct. 29, 19462,415,901 Nelson Feb. 18, 1947 2,442,018 Quarles May 25, 1948 FOREIGNPATENTS Number Country Date 705,329 Germany Apr. 29, 1941 117,787Australia Dec. 2, 1943 OTHER REFERENCES Strain et al.: Ind. 8: Eng.Chem., April 1939, pp. 382, 383.

Rothrock: Canadian Chem. and Process Industries, Sept. 1944, pp. 599 and600.

Wakeman: The Chemistry of Commercial Plastics, Reinhold, 1947, pp. 470and 471.

9. A PRESSURE-SENSITIVE ADHESIVE TAPE WOUND UPON ITSELT IN ROLL FORM,COMPRISING A FLEXIBLE BACKING AND, AS A COATING THEREON, A NORMALLYTACKY AND PRESSURE-SENSITIVE ADHEISIVE COMPOSITION COMPRISING OF A MAJORPROPORTION OF A SYNTHETIC RUBBERY POLYMER OF THE CLASS CONSISTING OFPOLYMERS OF ESTERS OF ACRYLIC ACID, ESTERS OF VINYL ALCOHOL, AND ETHERSOF VINYL ALCOHOL, HAVING SATURATED TERMINAL SIDE CHAINS OF AT LEAST FOURCARBON ATOMS WITH EACH OF SUCH TERMINAL SIDE CHAIN LINKED TO A SINGLECARBON ATOM THROUGH AN OXYGEN ATOM, AND A MINOR PROPORTION OF ACOMPATIBLE, OIL-SOLUBLE, HEAT-AND-OIL-REACTIVE PHENOL-ALDEHYDE RESIN.